In the formation of interconnects of a semiconductor circuit or in the formation of bumps, a method has recently come into use which involves performing plating on a substrate, such as a wafer, to form a metal film or an organic film on the substrate. It is common practice to form interconnects or bumps (protruding connecting electrodes) of gold, silver, copper, solder or nickel, or of a multi-layer laminate of these metals at predetermined sites on the surface of a wafer in which semiconductor circuits or fine interconnects that connect the circuits are formed, and to electrically connect the wafer to electrodes or TAB (Tape Automated Bonding) electrodes of a package substrate via the bumps. Such interconnects or bumps can be formed by various methods, including electroplating, electroless plating, vapor deposition, printing, etc. As the I/O number of semiconductor chip increases and the I/O pitch becomes narrower, electroplating, which can meet such a trend and can form a film at a high rate, has become a common method. A metal film as obtained by electroplating, the most commonly-used technique, is advantageous in high purity, high film-forming rate and easy control of the film thickness.
A typical plating apparatus will now be described with reference to FIG. 13. FIG. 13 is a schematic view of the typical plating apparatus. As shown in FIG. 13, this plating apparatus includes a plating bath 101 for holding a plating solution therein, an anode unit 107, and a substrate holder 104 for holding a substrate W. The anode unit 107 includes an anode 103. The substrate W and the anode 103 are disposed in a vertical position and are opposite each other in the plating solution held in the plating bath 101. A paddle 109, which reciprocates parallel to the surface of the substrate W to agitate the plating solution, is disposed between the anode 103 and the substrate W. By agitating the plating solution with the paddle 109, a sufficient amount of metal ions can be supplied uniformly to the surface of the substrate W.
The anode 103 is coupled to a positive electrode of a power source 105, while the substrate W is coupled to a negative electrode of the power source 105. The substrate W is plated by applying a voltage between the anode 103 and the substrate W. An overflow bath 106 is provided adjacent to the plating bath 101. The plating solution, overflowing the plating bath 101, flows into the overflow bath 106, and is returned into the plating bath 101 through a circulation line 120.
FIG. 14 is a perspective view of the anode unit 107, and FIG. 15 is a side view of the anode unit 107 shown in FIG. 14. As shown in FIGS. 14 and 15, the anode unit 107 includes a feeder belt 110 having a feeding portion 108 for passing an electric current to a central portion of the anode 103. The feeding portion 108 is in contact with only the central portion of the anode 103, and therefore the electric current flows from the central portion to a periphery of the anode 103 as shown by arrows in FIG. 15. Due to an influence of an electrical resistance of the anode 103, the current is lower at the periphery than at the central portion of the anode 103. Consequently, non-uniform electric current flows to the substrate W, and may adversely affect a uniformity of a thickness of a metal film formed on the substrate W.